Heat pipe

ABSTRACT

A heat pipe incorporates a novel wick including a single row of small-diameter rods disposed about the periphery of the heat pipe, each of the rods being wrapped in a spiral spacer wire, a cylindrical fine mesh screen located just inside the single row of rods, and a coarse support screen located just inside the fine mesh screen. A conventional working fluid such as sodium is employed in the heat pipe.

States Patent [72] lnventor Donald S. Trent 3,414,475 12/1968 Fibelmann165/105X Corvallis, Oreg- FOREIGN PATENTS [21] P 884,852 607,717 9/1948Great Britain 165/172 [22] Filed Dec. 15,1969 1451 Patented Apr. 27,1971 OTHER REFERENCES [73] Assignee The United States of America asrepresented Kem'me, J. E. HEAT PIPE CAPABILITY EXPERIMENTS, by theUnited States Atomic Energy Proceedings of Joint AEC/Sandia LaboratoriesHeat Pipe Commission Conference, Sandia Corp., Albuquerque, N.M.,10/1966 Vol.

1, pgs. 14 to 24, Microfische SC-M-66-623 Primary Examiner-Albert W.Davis, Jr. [5 Att0rney-Roland Anderson 1 Claim, 2 Drawing Figs. [52] US.Cl 165/105 ABSTRACT: A heat pipe incorporates a novel wick including[51] Int. Cl F28d 15/00 a single row of small-diameter rods disposedabout the [50] Field of Search 165/ 1 O5 periphery of the heat pipe,each of the rods being wrapped in a spiral spacer wire, a cylindricalfine mesh screen located just [56] References C'ted inside the singlerow of rods, and a coarse support screen UNITED STATES PATENTS locatedjust inside the fine mesh screen. A conventional work- 3,378,449 4/1968Roberts et al 165/105X s fluid such as Sodium is p y in the heat P p,IIIIIIIIIIIII HEAT mp1s CONTRACTUAL ORIGIN OF THE INVENTION Theinvention described herein was made in the course of, or under, acontract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to a heat-transferdevice. In more detail, the invention relates to a heat-transfer deviceof the type commonly known as a heat pipe. In still more detail, theinvention relates to a wick for a heat pipe.

The heat pipe is a high-flux, heat transport device which utilizes theevaporation, condensation, and surface tension of a working fluid toattain an exceptionally high thermal conductivity. A liquid isevaporated in a hot zone of the heat pipe, the gas thus formed flows toa cold zone where it is condensed, and the liquid flows back to the hotzone through a wick under the influence of capillary forces. The heatpipe can transfer over 500 times as much thermal energy per unit weightas can a solid thermal conductor of the same crms section. The heat pipeis simple, relatively inexpensive and operates silently and reliablyover a long lifetime. Thermal energy may be transferred to or from aheat pipe by radiation, convection or conduction; it can be used with avariety of energy sources, such as open flames, electric heaters ornuclear sources.

The only structural elements required to fonn a heat pipe are a closedouter shell, a porous capillary wick and a working fluid. The outer tubeof the heat pipe is normally circular in cross section and may be formedof glass or a variety of metals. The working fluid may be, for example,water, an organic liquid, ammonia, a molten salt or a molten metal.

Performance of a heat pipe is greatly dependent upon the wick design,since the wick is the capillary pump for fluid return to the hot zone.

The desirable features of a heat pipe wick include:

1. Low viscous drag in the capillary structure.

2. Small pore sizes at liquid-vapor interfaces to maximize capillarysuction pressure.

3. A fairly rigid barrier to separate the liquid and vapor phases sothat momentum interchange is minimized between the liquid and vaporphases.

4. Ease of wick saturation and wettability.

Several different types of wicks have been developed but, due largely tothe sometimes conflicting desirable features listed above, no singledesign appears ideally suited for all applications.

As pointed out in UCRL-50453, A Critical Review of Heat Pipe Theory andApplications," by Henry Cheung, wick design has undergone three majorstages of development, namely mesh, channel and screened channel. A meshwick consists of one or more layers of fine screen pressed closelyagainst the wall of the heat pipe; a channel wick is a series of smallaxial grooves in the'wall of the heat pipe and a screen channel includesa screen mesh fitting tightly over axial channels in the wall of theheat pipe. Recent variants on these wick designs include artery wicksand annular wicks. An artery wick includes a liquid return conduit whosewall is simply a fine screen and an annular wick is merely a fine screentube standing at a desired distance from the pipe wall.

It is accordingly an object of the present invention to develop a heatpipe of novel design.

It is also an object of the present invention to develop a heat pipewhich is easy to build.

It is another object of the present invention to develop a heat pipeincluding a wick which transmits substantial quantities of liquid withalow pressure drop and which is easy to rewet.

SUMMARY OF THE INVENTION These and other objects of the presentinvention are attained in a heat pipe incorporating a wick consisting ofa single row of small-diameter rods disposed around the periphery of theheat pipe, the rods being spaced one from the other and from the innerwall of the heat pipe by spacer wires wound spirally around the rods, afine mesh cylindrical screen located just inside the row of rods and acoarse support screen located just inside the fine mesh screen. Aconventional working fluid is also employed.

BRIEF DESCRIPTION OF THE DRAWING The invention will next be described inconnection with the accompanying drawing wherein:

FIG. I is a vertical cross section of a heat pipe constructed inaccordance with the present invention, and

FIG. 2 is a horizontal section thereof taken on the line 2-2 in FIG. I.

As shown in the drawing, the heat pipe includes an elongated,cylindrical tube l0, closed at both ends by end caps 11, and acylindrical wick 12 disposed within the pipe about its periphery andextending the length of the pipe. Wick 12 comprises a single ring ofsmall-diameter rods 13 spaced one from the other and from the inner wallof tube 10 by spacer wires 14 wound spirally around the rods. Justinside of the ring of rods 13 is a cylindrical wire mesh 15 havingsmall-diameter openings therein. A cylindrical coarse wire screen 16 isdisposed just inside of wire mesh 15 and serves to hold mesh 15 againstrods 13 and the entire wick 12 against the inner wall of tube 10. Itwill be appreciated that a working fluid, not shown in the drawing, isnecessary for operation of the heat pipe. The working fluid may be, forexample, sodium.

The operational characteristics of a sodium heat pipe of this type wereinvestigated. The heat pipe used inthese experiments was constructed oftype 316 stainless steel. The tube employed was Wt inches in insidediameter and 18 inches long. Inside the tube were a ring ofIIIG-inch-diameter rods wire wrapped with lO-mil wire to space themapart. Inside the solid rods two layers of IOU-mesh screen were followedby a support screen. One end of the tube was heated by an induction coiland the other end was cooled by two stainless steel water-cooledcollars.

Once the wick was made and inserted in the outer tube, the end caps, aninner tube for instrumentation and auxiliary filling and evacuatingassembly were welded on. The entire assembly was then vacuum degassed atl,200 F. Thirty-five grams of sodium was then distilled into the heatpipe, and the end tube was pinched off and welded shut.

The heat pipe was then operated in what may be considered a normal wayby applying power and turning on one of the cooling coils after all thesodium in the heat pipe was molten. After turning on the cooling coilthe power was slowly increased until dryout occurred. Until dryoutoccurred, the heat pipe behaved satisfactorily and in normal fashion intransferring heat from the hot end of the pipe to the cold end. Afterthese first tests, attempts were made to get to dryout prematurely andto see just how easily the wick does rewet after dryout. At a rate ofheat removal of L060 watts, the power was increased and a decrease inperformance of the wick was noticed. No settings were changed and afteran hour the wick had recovered and was operating at 1,140 watts. This isthe first experimental observation of spontaneous rewetting withouttaking any power decrease.

The dimensions within the wick and of the heat pipe are not critical.The rods must, of course, be small enough relative to the size of theheat pipe to provide sufficient vapor space. The width of the wire wrapmust be less than the diameter of the rod. The only limits on theoverall dimensions of the heat pipe are those applying to all heat pipesdue to the fundamental character of the device.

It will be understood thatthe invention is not to be limited by thedetails given herein but that it may be modified within the scope of theappended claim.

Iclaim:

II. A heat pipe comprising an elongated cylindrical housing containing awick consisting of a single row of small-diameter mesh screen locatedjust inside the row of rods, a coarse support screen located just insidethe fine mesh screen, said housing containing a low-melting metal asworking fluid.

1. A heat pipe comprising an elongated cylindrical housing containing awick consisting of a single row of small-diameter rods disposed aboutthe periphery of the housing, said rods being spaced from each other andfrom the wall of the housing by wires wrapped spirally around the rods,a cylindrical fine mesh screen located just inside the row of rods, acoarse support screen located just inside the fine mesh screen, saidhousing containing a low-melting metal as working fluid.